ZEITGEIST(a chess variant by John Cooper)
Zeitgeist uses most of the standard chess rule
set, but includes rules for simultaneous movement.

Before each turn, each player writes down his or her next move using
usual chess notation. When players signal that they are ready, both pieces
move at the same time. After simultaneous movement, some spaces might contain
two or three pieces. Conflicts of placement are resolved as follows:

Illegal pawn movement - Pawns can move forward, or capture diagonally.
A pawn must bounce back to the space it started from on that turn
if it captures an empty space (except through en passant), or moves
into a stationary piece's space. A stationary piece is any piece
that did not move in that same turn, or was forced to bounce.

Blocked movement - Only the knight can move through intervening
spaces occupied by stationary pieces; all other pieces attempting this
will bounce. Any piece may move through spaces occupied by pieces that
have moved in the same turn. As in normal chess, a player may not
castle through check. Any piece that bounces is treated as a stationary
piece for the rest of the turn.

Space occupied by exactly two pieces of the same color -
The active piece must bounce back to the space it launched from on that
turn.

Space occupied by exactly two pieces of different colors
- If both pieces captured into the space, both pieces are removed.
If only one piece captured into the space, the other is removed. If neither
piece captured into the space, all pieces retreat to launch spaces.

Space occupied by three pieces - All outnumbered and stationary
pieces are removed from play. (Example: I attempt to capture your stationary
pawn with my knight. meanwhile you use another pawn to capture into the
same space. The space now contains three pieces. My knight is outnumbered
by your two pawns, so it is removed. Your stationary pawn is also removed,
leaving your capturing pawn as victor.)

Pawn promotion - After resolving all placements, a player's
pawn that has reached its opposite row may promote. If both players are
promoting on the same turn, promotions are written down and then revealed
simultaneously.

Announce "check" - As in normal chess a player must give warning
if the opponent's king is in check, i.e. can be theoretically captured
on the following turn.

If a player's king is captured, or put in check for two consecutive turns,
that player loses. Note that it is entirely possible for both players to
lose at the same time.